In most of the biological systems studied, the biosynthesis of saturated fatty acids from acyl CoA derivatives involves participation of acetyl CoA carboxylase and fatty acid synthetase. We propose to investigate these enzyme systems in propionic acid bacteria because: 1) their presence has not been established, and 2) there exists the possibility that instead of acetyl CoA carboxylase (EC. 6.4.1.2), another enzyme system (e.g., transcarboxylase, EC. 2.1.3.1.) may carry out the formation of malonyl CoA required for fatty acid biosynthesis. Transcarboxylase has a major role in the formation of propionic acid during the fermentation of Propionibacterium shermanii. Its participation in fatty acid biosynthesis would not only provide a system hitherto undescribed but would also suggest a dual role for the enzyme in the intermediary metabolism of these bacteria. Investigations would be focused on mechanisms that might govern participation of transcarboxylase in these unrelated pathways. The proposed investigations may reveal the existence of different forms of transcarboxylase in nature which are path-specific: e.g., one form involved in propionate metabolism and the other constituting a part of the fatty acid synthetase complex. By virtue of its structural similarity to malonyl CoA, methylmalonyl CoA synthesized in vivo might be incorporated into fatty acids. One of the aims is to determine whether significant amounts of unusual and branched chain fatty acids are produced and whether they are incorporated into the membranes of these cells.